Shadow hunting: ASU urban climatologist helps us keep our cool


Scott Seckel

In an Arizona summer, the best parking spot is not the one by the door. It’s the one a quarter-mile away under a tree.

Ariane Middel, Arizona State University urban climatologist and hunter of shadows, knows this and has created a method to show us a cool way through the shade.

Middel, an assistant research professor in the School of Geographical Sciences and Urban Planning and a faculty affiliate of the Urban Climate Research Center, has developed a tool that will someday show pedestrians the shadiest — and hence coolest — route to their destinations. It also will tell planners and architects where they should throw shade.

She and a team of computer scientists used high-resolution Google Earth images to generate 180-degree “fish-eye” viewsto calculate whether a specific location would be in the sun or shade during a certain time of day. It’s a big-data approach that seizes upon Middel’s research.

Two years ago, she led a study to investigate thermal comfort at the university’s student union in Tempe. She and her team found that shade was the most important factor for comfort. Shade can make you feel up to 18 degrees cooler, while air temperature might only vary by 5 degrees. That got her interested in studying shade further.

“Keeping cool and staying comfortable is really difficult here in the desert in summer,” Middel said. “Out of all the variables that determine how comfortable you feel, shade is the most important factor, more important than air temperature, relative humidity, even the clothing that you’re wearing. Shade is the most important factor. That led me to look into the impact of shade on your thermal comfort when you’re outdoors.”

Video by Ken Fagan/ASU Now

The issue with shade is that it “travels” during the day, depending on the position of the sun. If you look at a north-south urban canyon, you will have shade from the building to the east in the morning, almost no shade at noon, and shade to the west in the afternoon. Middel wanted to look at the distribution of shade on campus and also assess the implications for thermal comfort.

To do it, Middel built a mobile weather station. (She calls it a mean radiant temperature cart. It looks somewhat like WALL-E.) Using the cart, Middel generated fish-eye views for the Tempe campus (one view every 5 yards) and then calculated thermal comfort for two times of day, 9 a.m. and 5 p.m., for a hot summer day last August.

“To make sure that the outputs from the model that we develop make sense, we have to ‘validate’ the outputs using real-world measurements,” she said. “I built a mobile platform that can measure thermal comfort (using three net radiometers that, simply put, measure the direct sunlight and the heat from surfaces that hit the human body from all sides).”

The next step was using the thermal comfort maps to find the most comfortable route from the Brickyard building on Mill Avenue to the Memorial Union. The algorithm suggested a more shaded route — longer than a direct route, but with much more shade.

“What we found measuring thermal comfort using the mean radiant temperature cart last summer was that thermal comfort varies a lot on campus,” Middel said. “It varies much more than air temperature.

“Mean radiant temperature, or thermal comfort, can be up to 60 degrees Celsius in the sun, but then it’s only 30 degrees Celsius in the shade. This is how you would feel this temperature as opposed to the air temperature you get from the weather report. There’s a huge variation due to trees, shade from buildings, shade from overhangs, different surface materials — all of this can be measured using this cart.”

Knowing the thermal comfort conditions at a very fine scale can be very helpful to architects and planners. It can be used in urban planning to assess whether an area has enough shade or maybe needs more shade.

University landscape architect Byron Sampson constantly hears pleas for more shade on campus. A tool like Middel is developing would be very useful to him, he said.

“If the interface will let the user know the shadiest route, we can learn where interventions need to take place and predict pedestrian movement through the university during different seasons,” Sampson said. “The paths do change and evolve over time.  As we look at the form of the built and open spaces in terms of development, we should be able to influence the architectural form to increase the shaded routes in areas where trees cannot be used to do the same.”

ASU urban climatologist Ariane Middel

This and top photo: ASU urban climatologist Ariane Middel uses her mean radiant temperature cart to measure thermal comfort of different spots on campus. Photos by Ken Fagan/ASU Now

As the Phoenix metro area becomes more urban and densities increase, shade will become the asset an area is judged by, Sampson said.

“If you look at the older neighborhoods with large trees (hint: more shade), the property values seem to be higher and stable,” he said. “This could also be a means to have interventions in poorer areas to increase the comfort level and significantly reduce the heat island in the Valley.”

Development on the tool continues. Middel has been using an existing thermal comfort model to translate the fish-eye information into thermal comfort. The model isn’t suited for large areas, because it only works for one fish-eye photo at a time.

“We are currently developing our own outdoor thermal comfort model,” she said. “Once working, (it) will be able to calculate thermal comfort at street level for any city where Google Street View data and basic weather station data are available.”

As Shakespeare said in “A Merchant of Venice,” “Some there be that shadows kiss; Such have but a shadow’s bliss.”